Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of photo-electronic elements (“elements”) such as rows of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has become a potential substitute. Examples of other elements/devices using organic materials include organic solar cells, organic transistors, organic detectors, and organic lasers.
An OLED is typically comprised of two or more thin organic layers (e.g., an electrically conducting organic layer and an emissive organic layer which emits light) which separate an anode and a cathode layer. Under an applied forward potential, the anode injects holes into the stack of organic layers, while the cathode injects electrons. The injected holes and electrons each migrate (under the influence of an externally applied electric field) toward the opposite electrode and recombine in the emissive layer under emission of a photon. Similar device structure and device operation applies for OLEDs consisting of small molecule organic layers and/or polymeric organic layers. Each of the OLEDs can be a pixel element in a passive/active matrix OLED display or an single element used as a general area light source and the like.
The construction of OLED light sources and OLED displays from individual OLED elements or devices is well known in the art. The displays and light sources may have one or more common layers such as common substrates, anodes or cathodes and one or more common organic layers sandwiched in between. They may also consist of photo-resist or electrical separators, bus lines, charge transport and/or charge injection layers, and the like. Typically, a transparent or semi-transparent glass substrate is used in bottom-emitting OLED devices.
White-emitting OLED-lighting devices can be generated by applying a continuous down-conversion layer on the light emitting side of a blue OLED. The down-conversion layer comprises of a color changing material, for example phosphor particles or organic dyes.
The phosphor layer can be structured as illustrated in the commonly-assigned US patent application entitled “Structured Luminescence Conversion Layer” filed on Oct. 31, 2005, bearing Ser. No. 11/264,516, and published as U.S. Publication No. 20070096634. Such structuring gives more flexibility in designing output spectra of down-conversion light sources. This flexibility allows finding a better compromise between efficiency and color rendering.
The mismatch of the refractive index between air and the OLED leads to most of the generated light being lost through total internal reflection into wave guiding modes and self absorption. Applying a phosphor layer or a scattering layer on the light emitting side of an OLED-device increases the output of OLEDs due to volumetric scattering mechanisms. Light extraction can also be improved by texturing the light emitting side of an OLED, for example by sand blasting or etching as described in a currently co pending commonly assigned US patent application entitled “Using Prismatic Microstructured Films for Image Blending in OLEDs” filed on Aug. 29, 2005, bearing Ser. No. 11/215,548 and published as U.S. Publication No. 20070046161.